ML19134A248
| ML19134A248 | |
| Person / Time | |
|---|---|
| Issue date: | 05/22/2019 |
| From: | David Rudland Division of Materials and License Renewal |
| To: | |
| Shared Package | |
| ML19136A264 | List:
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| References | |
| Download: ML19134A248 (7) | |
Text
Control Rod Drive Mechanism (CRDM)
Thermal Sleeve Flange Wear Analyses May 22, 2019 David L. Rudland Senior Technical Advisor for Materials Division of Materials and License Renewal U.S. NRC Office of Nuclear Reactor Regulations
2 Purpose of Sleeve:
Guides rod cluster control assembly (RCCA) drive rods into the head penetration tubes during reactor vessel head installation Provide thermal shielding of the head penetration tubes Wear remnant can cause control rod to stick
- Information on wear rates uncertain
- 95% at 0.03/year, 99% at 0.04/year from limited US data -
French saw 0.12/year
- Unknown if wear was uniform or location specific
- Wear will continue until failure occurs
- Deterministically thermal sleeve failures is predicted after 12.5 years with multiple sleeve failures - possible large safety impact
- Using probabilistic analyses with risk insights provide a more appropriate prediction of behavior and treatment of uncertainty
- Impacts on defense-in-depth and safety margins?
3
- LIC-504 - Integrated Risk-Informed Decision-Making Process for Emergent Issues
- Compliance with Existing Regulations
- Consistency with the Defense-in-Depth Philosophy
- Maintenance of Adequate Safety Margins
- Demonstration of Acceptable Levels of Risk
- Implementation of Defined Performance Measurement Strategies
- Cross-discipline integrated review team -
materials, systems, risk 4
This presentations focus
5 Materials staff focused on development of initiation event frequency, i.e., CRDM thermal sleeve failure Risk and systems staff focused on core damage frequency analyses using material staffs input Many unknowns, e.g., wear rate, etc lead to conservative assumptions Number of failures Probability of failure between outages Wear rate, in/yr Probability
Conclusion:
Five principles were met, but additional data needed to confirm assumptions - smart sample (OpESS) inspection recommended
- 62% of most susceptible plants were sampled by Regions 6
Licensees followed guidance appropriately, NRC assumptions were conservative
- Effort completed in 7 months using integrated decisionmaking and integrated review team
- Five principles of integrated risk-informed decisionmaking satisfied
- Smart sample inspection verified NRC analysis assumptions
- Will continue to monitor industry operating experience on this issue 7